The present invention relates generally to nonlinear optical materials, and more specifically to improving the mechanical properties and nonlinear optical performance characteristics of gallium selenide crystals.
Nonlinear optical (NLO) materials, particularly NLO crystals, are increasingly valuable for such uses as frequency conversion. Frequency conversion converts, or shifts, monochromatic, generally laser, light to a useful higher or lower frequency not directly available from a conventional laser source. Frequency conversion using NLO materials can be achieved through such NLO methods as second harmonic generation (SHG), optical parametric generation and difference frequency conversion with tunable lasers.
Second harmonic generation is generally used to double the frequency (half the wavelength) of far-infrared carbon dioxide lasers. Optical parametric generation is generally used to double the wavelength of various near-infrared solid-state lasers. The frequency conversion process in a crystal material is only efficient along certain crystallographic directions known as "phase matching" directions.
Several NLO crystals such as thallium arsenic selenide (TAS), silver gallium selenide (AgGaSe.sub.2) and zinc germanium phosphide (ZnGeP.sub.2) are being developed as potential candidates for frequency conversion. These crystals are capable of generating the second and higher order harmonic wavelengths necessary for frequency conversion. Second and higher order harmonic wavelengths are also necessary for various other applications in the mid-IR wavelength region. For example, TAS is used for frequency conversion of a CO.sub.2 laser from wavelengths of 10.6 .mu.m to 5.3 .mu.m. However, the use of TAS crystals is limited to low power applications due to its low thermal conductivity. In high power applications, a thermal lensing effect (the index of refraction increases with intensity and temperature in these materials, creating a thermal lensing effect at higher intensities and temperatures) causes cracks in the crystal. A problem with AgGaSe.sub.2 is that it has an absorption peak at .about.2 .mu.m, the desired pump wavelength for applications in the 3-5 .mu.m atmospheric window.
Gallium selenide (GaSe) is a highly efficient NLO material with a wide transmission range from 0.65 to 18 .mu.m and has a substantial potential for use in tunable laser light sources in the mid-infrared range. It can be phase matched for both type I and type II SHG and has been used for difference frequency generation and for optical parametric generation.
A particular advantage of GaSe is its potential applicability for use in high power applications. Not only does GaSe exhibit high thermal conductivity, but its optical axis is perpendicular to its preferred direction of thermal conductivity, thereby desirably increasing its ability to conduct thermal energy away from its optical axis.
The major stumbling block preventing widespread use of GaSe is that it cleaves easily along planes perpendicular to the crystal c-axis, making cutting and polishing optical faces difficult.
Thus it is seen that there is a need for GaSe crystals having improved structural properties suitable for use as NLO crystals, particularly for GaSe crystals having improved fabricability.
It is, therefore, a principal object of the present invention to provide improved GaSe crystals which exhibit a significant reduction in cleaving tendency over prior art GaSe crystals.
It is another object of the present invention to improve the second harmonic generation efficiency of GaSe crystals.
It is a feature of the present invention that GaSe crystals made according to the teachings of the present invention are of a higher optical quality than pure GaSe crystals.
It is another feature of the present invention that the bulk optical transmission characteristics of GaSe crystals made according to the teachings of the present invention are not affected by use of the invention.
It is an advantage of the present invention that it allows cutting and polishing of optical faces of GaSe crystals along arbitrary directions.
It is another advantage of the present invention that GaSe crystals made according to the teachings of the present invention have higher structural integrity and stability than prior art GaSe crystals.
It is a further advantage of the present invention that the ability of GaSe crystals to withstand high thermal energies is not lessened by modifications made according to the teachings of the present invention.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.